1. Field of the Invention
The present invention relates to a light emitting element capable of being driven by alternating current drive, and to a light emitting device containing the light emitting element as a portion of the light emitting device.
2. Description of the Related Art
A light emitting element is made from an anode, a cathode, and an organic compound layer sandwiched by the anode and the cathode. Light emission develops by carriers injected from both electrodes (electrons and holes) recombining within the organic compound layer and generating electrical excitation states.
However, electric charge accumulates in the organic compound layer in this type of light emitting element when using direct current drive, in which a fixed direction bias is always applied, and therefore there arises a problem in that the light emission lifetime and brightness are reduced.
Reports of using alternating current drive, in which a driver voltage applied to the light emitting element during light emission and a reverse bias that has an inverse polarity with respect to the driver voltage are applied alternately, are made as disclosed, for example, in JP 10-172760 A, JP 11-8064 A, and the like.
This is because the electric charge that accumulates in an inner portion of the organic compound layer is relieved by alternately applying voltages having different polarities to the organic compound layer by alternating current drive, and therefore reductions in the light emission lifetime and brightness can be suppressed.
However, in the case of light emitting elements in accordance with alternating current drive, the light emitting element normally has a laminate structure composed of an anode, an organic compound layer, and a cathode, and therefore light emission can be obtained only when a positive voltage is applied from the anode side and a negative voltage is applied to the cathode side, namely when a forward bias is applied. That is, the light emitting element does not emit light when a reverse bias is applied using alternating current drive.
Thus, when an effective light emitting time is reduced, display becomes dark. Therefore, when a high voltage is applied in order to maintain a predetermined brightness, there arises a problem in that degradation of the light emitting elements proceeds.